This invention relates in general to filters and methods for manufacturing filters, and more specifically to tubular microporous filtration elements operating with a forced inside-to-outside fluid flow.
Although filter media have been formed from a wide variety of materials, including paper, woven fabrics, and beds of fine granular materials such as sand or charcoal, it is convenient to classify filter media as either depth filters or screen filters. The terms "depth" and "screen" describe the mechanisms by which each of these types of filter media retain particles and remove them from the fluid passing through the media. A fibrous depth filter, such as an automobile oil filter, has a porous structure defined by a random array of fibers that are intertwined and interconnected. The spaces between the fibers form tortuous flow passages of variable size in which the particles become trapped as the fluid flows through. Because of the inherent variability and randomness of the pore size and structure, depth filters retain particles having a wide range of sizes, while allowing a certain percentage of particles of all sizes to pass through the filter media.
In contrast, screen or absolute filters remove particles from the fluid primarily by a sieving mechanism at the upstream surface of the filter. A screen filter is a porous matrix having a precisely defined pore size. A window screen is a common example of what has come to be called an absolute filter. With such a filter, the filtration is "absolute" since any particles larger than the hole size are retained on the filter surface. Thus the pore size of a screen filter establishes a cut-off on the size of the particulate matter found suspended in the filtered fluid. It should be noted, however, that if the particulate matter is deformable, as for example bacteria, then the relationship between the pore size and the size of the particles screened by the filter is somewhat more complex. As used herein, the term "filtration" will refer to the action of a screen or absolute filter and may be defined as the removal of particles or bacteria from a fluid that are larger than the pore size of the filter material.
Besides differing in their retention efficiency, that is, the ability of the filter media to remove particles of varying size from the fluid, depth and screen filters also differ in their capacity to hold particles. Depth filters generally have a higher particle holding capacity since they retain particles both on their surface and within the pores themselves. Thus the dirt-holding capacity of a depth filter for a given size of particles will depend on such factors as the thickness of the filter media and the density of the filter media and hence its pore size distribution. Screen filters generally have a lower particle holding capacity since they retain particles only on the upstream filter surface. When this upstream surface becomes plugged by the retained, filtered particles, it becomes ineffective as a filtering element and must be replaced.
In many applications, it is necessary to totally remove particles having dimensions in the submicrometer range. For this purpose it is well known in the art to use a thin polymer layer that is rendered highly porous with a highly uniform pore size. Such layers are commonly termed microporous filtration materials or membranes.
One characteristic of such microporous filtration materials is that they are extremely fragile and easily rupture when subjected to deformation due to rough handling, bending or fluid pressure. Since even the most minute crack or break will destroy their effectiveness, it is necessary to use extreme care in their manufacture and use. Another characteristic of microporous membrane materials is that most of such materials expand by about six percent when wet. This characteristic is commonly termed "wet growth". These materials also exhibit a significant degree of sliding friction.
Microporous filtration materials find many uses in industry, science and education. A common industrial application is the "cold" sterilization of pharmaceuticals and the stabilization of alcoholic beverages. In cold sterilization, the filtration material has a sufficiently small pore size to block the passage of all bacteria present in the unfiltered fluid supplied to the upstream side. In the production of alcoholic beverages, the removal of bacteria yeast and molds stabilizes and clarifies the beverage. In the production of many pharmaceutical products, the removal of bacteria is an essential step for obvious health reasons.
To be commercially practical, the microporous filtration material must process a large volume of a fluid in a reasonably short period of time. It is therefore standard practice to apply a positive pressure to the unfiltered fluid which forces the fluid through the filtration material at an acceptable rate, particularly when filtered particles accumulate or "cake" on the upstream surface.
In addition to a high production rate, it is of great importance to be able to test the integrity of the filtration system before and after a production run. Without such a test capability, for example, a batch of an antibiotic valued at tens of thousands of dollars could be run through a defective or improperly installed filtration element before the defect is detected through a test of the end product. An important advantage of microporous filtration material is that their integrity can be readily checked both before and after a production run by a "bubble point" test. To perform the test, the filtration layer is wet with a liquid which is then removed from the upstream surface of the layer only. The region above the filter media is filled with a regulated pressure gas such as air or nitrogen. The pressure is gradually increased until bubbles of the gas appear in the downstream liquid. The pressure at which the bubbles first begin to appear is commonly termed the "bubble point". At the bubble point the pressure of the gas exceeds the capillary attraction for the liquid held in the pores of the filtration layer and is therefore a direct measurement of the effective diameter of the pores and thus their filtration efficiency and integrity. If the filtration layer is broken, even microscopically, or if the filtration layer is improperly installed, bubbles will appear immediately at the point of the break or leak since there is no capillary resistance to the gas flow. A porous material having extremely large pores and a negligible flow resistance will have a zero bubble point.
Microporous filtration media are most commonly used in the form of discs. Because of the fragility of the filtration material and the desirabilit of operating the filtration process with an applied pressure, the discs are usually used in conjunction with a support screen and holder commonly termed a "pie plate". To increase the effective filtration surface and thereby accommodate larger volumes of raw fluid and increase the operational life of the filtration system, it is well known in the art to utilize a large number of discs (frequently in excess of 60) and their associated support structures arranged in a stack. Although a filtration system of this type has proven to be highly successful, it has a serious drawback in that it usually requires two to three hours of careful work to assemble such a stack, particularly when all of the elements must be maintained in a sterile condition.
It has been found that a more convenient arrangement is to form the filtration layer into a tubular structure which can be quickly inserted into a suitable housing for directing the fluid flow radially through the walls of the tube. Besides avoiding the laborious stack assembly, the tubular configuration also has the advantage of presenting a relatively large surface area to the input fluid which results in an increased production rate and a prolonged operational life for the filtration media.
Tubular constructions of this type are known both in the field of microfiltration and in conventional depth filter systems such as reverse osmosis filter membranes commonly used in water desalinization and fluid concentration processes. Representative reverse osmosis filter elements and systems are described in U.S. Pat. No. 3,578,175 to Manjikian, and U.S. Pat. No. 3,715,036 to Hamer. The reverse osmosis filter, however, is not effective for filtration, especially microfiltration to achieve cold sterilization. First, it does not totally remove particles above a given size. Second, it suffers from media migration, that is, the constituent material of the filter breaks off, or sheds, and therefore contributes particulate contamination to the filtrate. Third, it is susceptible to the release of retained particles under "shock", a sudden change or reversal in the fluid pressure of the system due, for example, to valving.
Tubular microporous filtration elements heretofore known in the art are characterized by a perforated support tube fabricated from a structural material such as metal which is wrapped with an overlying support layer of a material such as woven nylon fabric, a layer of the microporous filtration material, and an outer protective layer of a porous material such as the nylon fabric. The structure is adapted to an outside-to-inside fluid flow with the perforated metal tube providing mechanical support for the filtration layer. The nylon support layer avoids the cost and technical difficulty of machining or etching a suitable number of extremely small diameter perforations in the support tube. This construction also reduces the potential for damage to the fragile filtration layer during manufacture since it is wrapped directly onto the nylon-lined core.
The tubular filtration elements of this prior construction are replaceably mounted in a suitable housing that directs the fluid in an outside-to-inside flow direction. A clamp acting through an O-ring positioned inside the filtration element secures the element to a surrounding portion of the housing. The reliability of this mounting system depends on the skill of the user in properly positioning and clamping the filtration element.
Although the tubular filtration element is significantly more convenient to use than the stacked disc system, it nevertheless has certain shortcomings. First, conventional tubular filtration elements are not "in-line" steam-sterilizable, that is, sterilizable by the passage of pressurized steam (typically at temperatures in excess of 121.degree. C and pressures in excess of 15 psi) through the production apparatus including the filtration unit. The pressurized steam causes an outward expansion of the filtration layer due to wet growth. Since the fluid flow direction and the pressure differential are outside-to-inside, the wet growth results in the formation of wrinkles in the filtration media. Each wrinkle is a weakened stress area within the filtration layer which is highly susceptible to rupture. Since in-line sterilization is not feasible, the filtration unit must be disassembled, autoclaved, and reassembled under aseptic conditions prior to each production run. This procedure is time consuming, costly, and increases the likelihood of inadvertant bacterial contamination.
To avoid the wrinkling problem and achieve an inline sterilization capability, it is desirable to utilize an inside-to-outside flow direction. This arrangement, however, presents the problem of providing the proper downstream support for the filtration layer. The downstream support must be mechanically strong while having a high degree of porosity, have a small pore structure to prevent the protrusion of the filter layer into the pores, and not suffer from media migration or otherwise contribute particulate contamination to the filtered fluid. An inside-to-outside flow direction also raises a manufacturing problem since there is no perforated core of structural material on which to form the filtration layer. The filtration media must therefore be wrapped into a tubular form on a suitable cylindrical object such as a mandrel and removed from the mandrel. The relatively high sliding friction of the filtration material, however, prevents the sliding disengagement of the mandrel and the fragile filtration layer without the use of sufficient force to break or weaken the layer.
It is therefore a principal object of this invention to provide a tubular filtration element that reliably removes particulate matter and bacteria in excess of a predetermined size from a fluid flowing from the inside to the outside of the filtration element and is in-line steam sterilizable.
A further object of the invention is to provide a tubular filtration element having the advantages described above which is conveniently loaded and unloaded in a filtration system.
Yet another object of the invention is to provide a tubular filtration element which does not develop stress areas during operation that are susceptible to rupture and therefore has a high degree of reliability and a long operational life.
Still another object of the invention is to provide a tubular filtration element which is mechanically strong, has a relatively low flow resistance, does not shed particulate contaminants into the filtered fluid, and may be tested as to its integrity both before and after use.
Still another object of this invention is to provide a tubular filtration element which meets government regulations for the processing of food and drugs and has a relatively low cost of manufacture.
Another object of this invention is to provide a method for continuously and rapidly fabricating helically wrapped tubular filtration elements embodying the features and advantages of the invention without rupturing or weakening the fragile microporous filtration material.